Method and device for chemical reaction between a gas and at least one compound in solution carried out in the presence of a solid catalyst

ABSTRACT

A gas/liquid reaction with a liquid containing at least one chemical compound in solution which can react with a gas in the presence of a solid catalyst maintained in suspension in the liquid, the reaction being carried out with separation (recovery) of said solid catalyst by filtering. Recirculating a fraction of the reacted solution without involving any additional pump or ejector, wherein the recirculated fraction provides for a primary filter for reacted solution being tangentially skimmed over.

The object of the present invention is:

-   -   a method for carrying out a chemical reaction between a gas and        at least one compound in solution, whereby the method is carried        out continuously in a stirred reactor in the presence of a solid        catalyst maintained, in a dispersed state, in suspension within        the solution containing said compound; said method including the        separation of said catalyst from said reacted solution;    -   a device suitable for carrying out said method.

The present invention has, more particularly, been developed, withrespect to the production of hydrogen peroxide (H₂O₂) using theanthraquinone method, for the catalytic hydrogenation of the workingsolution. Its field of application is however in no way limited thereto.Those skilled in the art will readily understand from the following thatthe field of application of the invention relates to any gas/liquid(G/L) reaction, liquid (L) containing, in solution, at least onechemical compound which can react with gas (G) in the presence of asolid catalyst which is held in suspension in said liquid (L).

According to the prior art, the execution of gas/liquid (GIL) reactionmethods in the presence of a solid catalyst has been described; saidsolid catalyst being fixed (referred to as a fixed bed catalyst) ordispersed, in suspension within said liquid. In this second case, theproblem of separating said catalyst from said reacted liquid arises.Such separation can be carried out using hydrocyclones. This is notablyillustrated in application WO 98/282 25. Such separation can also bebased on filtering operations.

The invention relates more precisely to gas/liquid (G/L) reactions,liquid (L) containing, in solution, at least one chemical compound,which can react with gas (G), in the presence of a solid catalystmaintained in suspension in said liquid (L), said reaction methods beingcarried out with separation (recovery) of said solid catalyst byfiltering.

To facilitate, on the one hand, the disclosure below of the presentinvention, as regards the aspects of its method and device, -and on theother hand, the understanding of said invention, it is proposed, withreference to appended FIG. 1, that the prior art closest to theinvention, as disclosed in general terms above, now be described indetail. An improvement to this prior art is proposed according to theinvention.

According to said prior art, a solution WS (containing at least onecompound which can react with gas G used) is treated continuously inreactor 1 with a gas G in the presence of a solid catalyst. Saidsolution WS is circulated at flow Q by pumps P1 and P2.

The treatment in question can indeed consist in hydrogenating (G=H₂) theworking solution (at least one derivative of the anthraquinone dissolvedin a complex mixture of organic solvents) which is used for theproduction of hydrogen peroxide (H₂O₂) in an anthraquinone cyclicmethod; said hydrogenation is carried out in the presence of apalladium-based catalyst which is supported on aluminium oraluminosilicate grains.

Solution WS is efficiently treated in stirred reactor 1 within whichsolid catalyst and reactive gas (G) are dispersed and maintained insuspension.

When exiting reactor 1, the catalyst is separated from treated solutionWS′ using a series of filters, including in succession:

-   -   at least one primary filter F1 generally of the type having a        carbon or sintered metal candle (coarse filter with a porosity        adapted to the porosity of the catalyst grains to be retained,        for example a porosity of approximately 5 μm). In the        advantageous variation shown, two such primary filters F1,        mounted in parallel, are used;    -   at least one secondary filter generally of the cartridge filter        type with a finer porosity (for example, approximately 0.5 μm),        for stopping finer particles which have inexorably been        generated as a result of the abrasion of the catalyst grains, as        a result of the considerable agitation which is necessary in the        reactor. In the advantageous variation shown, two such secondary        filters, F2 and F3, mounted in series, are used.

The total flow Q of the solution entering reactor 1 in order to betreated is sent towards and passes through said filters F1, F2 and F3.Said flow Q effectively passes through single operational filter F1(single filter F1 used or single operational filter F1 of a set of atleast two filters mounted in parallel) or is divided between at leasttwo operational filters F1 of such a set of filters. In the latter case,it is considered that flow Q passes through said operational filterswhich are considered as a whole.

Primary filters F1 are sequentially washed in a counter-currentdirection to prevent pores thereof from becoming blocked and to returnthe retained catalyst grains into reactor 1. This sequential washinginvolves a flow q taken, according to the variation shown, from flow Qof treated and filtered solution WS″.

In addition to sequential washing, said primary filters F1 mustregularly (once every three weeks as regards H₂O₂ production) undergochemical washing (acid and/or basic) in order to remain clean.

Said primary filters F1 are largely stressed insofar as, on the whole,all of flow Q of treated solution WS′ passes therethrough and thechemical washes are relatively aggressive treatments. They must bechanged frequently.

The same applies to secondary filters F2 and F3. In fact, insofar as theeffectiveness of the reactor is linked to the stirring force inside it,the abrasion of the grains of the catalyst used is relatively large.

Carrying out this method of the prior art leads to technical problems,notably the four given below:

-   -   the solid catalyst used, carried by flow Q, tends to collect on        the primary filters. Said thus collected catalyst is no longer        active in the reactor and is responsible for a head loss that is        detrimental for maintaining said flow Q;    -   the chemical cleaning and the replacement of the candles in the        primary filters are costly operations;    -   the replacement of the cartridges in the secondary filters is        also a costly operation;    -   in view of the relation between flows q and Q, each        counter-current wash upsets the operation of the reactor as a        result of solution WS″ and catalyst being returned into said        reactor.

The invention has been developed, in relation to the foregoing withreference to said FIG. 1, in order to overcome said technical problems.

According to its first object, said invention therefore relates to amethod for causing a chemical reaction between a gas and at least onecompound in solution, the method being carried out continuously in astirred reactor in the presence of a solid catalyst maintained, in adispersed state, in suspension within the solution containing saidcompound.

Typically, the method comprises:

-   -   supplying the reactor, at its bottom portion, with said solution        to be reacted and with said gas;    -   bringing into contact, inside said reactor, said solution, said        gas and said catalyst for the purposes of carrying out said        chemical reaction;    -   recovering, at the top portion of said reactor, the reacted        solution;    -   filtering said solution by passing it successively through at        least one primary filter and at least one secondary filter;    -   recovering the reacted solution which is thus rid of said        catalyst;    -   sequentially backwashing said primary filter(s) with a pulsed        flow taken from said filtered, reacted solution and/or from the        supply solution to be reacted; said backwashing being designed        to return the catalyst retained on said primary filter(s) into        said reactor.

These successive stages of the method of the invention are the same asin the prior art as schematically shown in FIG. 1.

Characteristically, said method of the invention further comprises therecirculation of a fraction of the reacted solution towards the bottomportion of said reactor, at a flow which is greater than the flow forsupplying said reactor with the solution to be reacted; saidrecirculation being provided by sufficient stirring inside the reactorand said fraction of the reacted solution being redirected, for saidrecirculation, at said primary filter(s) and thus providing fortangential skimming over said primary filter(s).

Characteristically, according to the method of the invention, tangentialskimming over said primary filter(s) is provided by a large flow ofreacted solution recirculated at the bottom portion of the reactor; thestirring operation itself inside said reactor providing for saidrecirculation with a large flow.

In a non-limiting manner, it is specified at this point that thisrecirculation flow of the reacted solution is generally 3 to 10 timesgreater than the flow for supplying the reactor with the solution to bereacted. It is advantageously 5 to 10 times greater than said supplyflow.

The key point of the method of the invention as it is carried out is thestirring operation inside the reactor. Said stirring is typicallyresponsible for the extent of gas/liquid/catalyst contact. Furthermore,it must provide, in a more original manner, for the expected pumpingeffect (the recirculation of a fraction of the reacted solution).

According to an optimised manner of carrying out the method of theinvention, said stirring must provide for:

-   -   the dispersion of the gas and the catalyst within the solution        (optimised dispersion for optimisation of the above-mentioned        contact);    -   the expected pumping effect (for recirculation); and    -   the internal recirculation (inside the reactor) of the solution.

In fact, it is quite appropriate that the dwell time of said solutioninside said reactor is sufficient for the gas/liquid reaction to takeplace correctly.

In view of the above, the person skilled in the art has alreadyunderstood the entire significance of the method of the invention. Itsembodiment offers a number of advantages. Notably:

-   -   a) there is a considerable increase in the reactor supply        flow—it now consists in both the flow for supplying the solution        to be reacted and the recycled solution flow (generally 3 to 10        times greater than said supply flow)—the disruptive effect of        sequentially backwashing the primary filter(s) is considerably        reduced or even eliminated;    -   b) mixed filtering carried out at said primary filter(s). A flow        of treated solution corresponding to the reactor supply flow        passes through the filter(s) and a larger flow of treated        solution skims over the filter(s). This skimming minimises any        accumulation of catalyst grains and fines, continuously cleans        said filter(s) and recirculates said grains and fines towards        the reactor. Thus, the catalyst concentration inside the reactor        is optimised; the life span of the primary and secondary        filter(s) is considerably increased (the frequency of chemically        washing the candles of the primary filters is largely decreased        as is the frequency of changing the cartridges of the secondary        filters). As regards the hydrogenation of the working solution        of an anthraquinone cyclic method for producing H₂O₂, an annual        change and chemical wash have proved to be sufficient; the        problem of the above-mentioned head loss (as a result of the        treated solution passing through the collected catalyst) no        longer exists;    -   c) in view of the recirculation of a fraction of the reacted        solution, the reaction carried out inside the reactor can be        controlled more easily; notably with regard to the formation        of-by-products;    -   d) it is possible to optimise stirring inside the reactor to        provide for a high level of distribution of the various liquid,        solid, and gas components while limiting the abrasion of the        catalyst;    -   e) with reference to points b) to d) above, catalyst consumption        is reduced (by comparison to the consumption in a method of the        type schematically shown in FIG. 1).

The method of the invention is particularly suitable for carrying outhydrogenation. In such contexts, the gas used consists of hydrogen or,in any case, contains hydrogen; the solution used contains at least onecompound which can react with said hydrogen.

The method of the invention is particularly suitable for thehydrogenation of at least one derivative of anthraquinone in solution inthe working solution as regards the preparation of hydrogen peroxide(H₂O₂). This is notably specified in applications WO98/28225 and EP016622. In said context, the embodiment of the invention has enabled theapplicant to obtain extremely interesting results. Depending on thecomposition of the working solution, she obtained hydrogenation levelswhich were as high as 10 g/l to 15 g/l (g of H₂O₂/litre of workingsolution), while limiting the formation of by-products.

There now follows the description, in general terms, of the secondobject of the present invention, namely a device which is suitable forcarrying out the method described above (first object of saidinvention).

Said device typically comprises:

-   -   a reactor equipped with stirring means, solution supply means,        gas supply means and means for delivering the reacted solution;        said solution and gas supply means being arranged at the bottom        portion of said reactor whereas said means for delivering the        reacted solution are arranged at the top portion;    -   a filtering assembly mounted, at the outlet of said reactor, on        said means for delivering the reacted solution and including at        least one primary filter (generally, a set of at least two        primary filters mounted in parallel such that it is possible to        pass from one filter to another filter without interrupting the        process) and at least one secondary filter (generally several        secondary filters mounted in series);    -   means for carrying out sequential backwashing of said primary        filter(s) with a pulsed flow q taken downstream of said        filtering assembly from the filtered reacted solution and/or        upstream of said reactor from the solution to be reacted.

In that respect, the device of the invention is of the prior art type,such as schematically shown in FIG. 1.

Characteristically, said device of the invention further comprises meansfor recirculating a flow of reacted solution, with tangential skimmingover the primary filter(s), towards the bottom of said reactor and thestirring means of said reactor provide, themselves, for saidrecirculation.

Said means consist of an adequate circuit which successively providesfor:

-   -   a fraction of the reacted solution being redirected at the top        outlet of the reactor;    -   said redirected fraction tangentially skimming over operational        primary filter(s);    -   recirculating said redirected fraction at the bottom portion of        the reactor.

This fraction can be reintroduced into said reactor, independently, orotherwise, of said reactor being supplied with the solution to bereacted. According to an advantageous variation, a mixture of saidrecirculated fraction and said solution to be reacted is supplied in asingle operation at the bottom portion of the reactor; said mixturehaving been formed upstream.

The stirring means in the reactor, which can provide for said stirringand recirculation of the redirected fraction, can also advantageouslyprovide for internal recirculation. They consist advantageously of amulti-stage stirrer (generally with at least three stages).

With respect to an optimised alternative embodiment, said stirring meanscomprise:

-   -   a dispersion device, which operates in the bottom portion of the        reactor;    -   a middle stage, which can provide for internal recirculation        inside the reactor and help pump the reacted solution towards        the primary filter(s);    -   an upper stage, which can ensure pumping of said reacted        solution towards said primary filter(s).

Said dispersion device provides for a high level of dispersion of thecatalyst and the gas within the solution containing the reagent(s) insolution.

Said middle stage (another device) provides for some pumping but aboveall for internal recirculation of the reaction mixture in the reactor(it allows the dwell time of said mixture in said reactor to beincreased).

Said upper stage (another device) provides for pumping the reactedsolution towards the primary filter(s). The flow provided must be largeso as to constantly recirculate the catalyst towards the bottom of thereactor.

According to an alternative embodiment, for the purposes specifiedabove, said middle and upper levels include blades which are inclinedupwards and downwards respectively.

The device of the invention as described above is advantageouslyarranged in a hydrogen peroxide production loop, using the anthraquinonecyclic method, at the reactor for hydrogenating the working solution.

It is now proposed that the invention be reconsidered, as regards twoaspects relating to method and device, with reference to the appendedfigures.

FIG. 1 illustrates the prior art closest to said invention. Referencewas made to it in the introduction of the present text.

FIG. 2 illustrates a preferred variation for carrying out saidinvention. It must be considered in conjunction with said FIG. 1.

The solution to be reacted, WS, is supplied at a flow Q using pump P1.It is introduced into the bottom of the reactor after having been mixedwith a fraction of the reacted solution WS′, this fraction beingrecirculated at a flow Q′. It has been seen that advantageously Q′≧3 Q.

Furthermore, the mixture sequentially contains flow q of treated andfiltered solution WS″. This flow q was used to sequentially wash primaryfilter(s) F1.

Said mixture is made to react in the bottom portion of reactor 1 withgas G.

Stirring means 2 of said reactor 1 are of the optimum type specifiedabove. They include a lower stage or dispersion device 2′, a middlestage 2″ which principally provides for recirculation of the solution inreactor 1 and an upper stage 2′″ which principally provides for pumpingreacted solution WS′.

It can be considered, schematically, that pump P1 provides for thecirculation of flow Q of solution WS whereas upper stage 2′″ of stirringmeans 2 provides, principally by itself, for the loop circulation offlow Q′ of solution WS′. A second pump P2 is no longer needed accordingto FIG. 1.

Mixed filtering is carried out at primary filters F1. Flow Q of solutionWS′ passes through said filters F1 while flow Q′ of said solution WS′tangentially skims over said filters F1. Said flow Q of solution WS′then passes successively through secondary filters F2 and F3. Flow q istaken from the filtered, reacted solution in order to carry outsequential counter-current washing of filters F1. According to anothervariation, said flow q could have been taken from flow Q of supplyingsolution WS to be reacted (before its mixing with the recirculatedsolution).

The amount of catalyst carried beyond primary filters F1 is minimised.

It is proposed, finally, that the invention be illustrated by thefollowing example.

Said invention has been carried out, as illustrated in FIG. 2, duringthe catalytic hydrogenation (G=H₂, catalyst=supported Pd) of a workingsolution (WS) used for the production of hydrogen peroxide by theanthraquinone cyclic method. Said production was carried out at pilotstage with a 170-liter reactor.

Said reactor was equipped with a three-level stirrer, comprising frombottom to top:

-   -   a Rushton turbine with 6 fixed blades;    -   a turbine with 6 blades that can be variably inclined between        30° and 60°;    -   a “Sabre” propeller.

The flows were regulated as follows:Q _(WS)=0.8 m³/hQ _(WS)=3.5 m³/hq _(WS″)=2.8 m³/h, corresponding to pressure flushing of a volume of 8 lof liquid for 10 sec every 30 min.

It is under the above-specified conditions that the good results givenabove have been obtained:

-   -   a high degree of hydrogenation of 10 to 15 g/l;    -   annual chemical washing and replacement of the primary filters;    -   increase of the life span of the secondary filters.

1. A method performed in a stirred reactor for carrying out a chemicalreaction between a gas and at least one compound in solution and in thepresence of a solid catalyst maintained, in a dispersed state, insuspension within the solution containing said compound; said methodcomprising: supplying said reactor, at its bottom portion, with saidsolution to be reacted and with said gas; bringing into contact, insidesaid reactor, said solution, said gas and said catalyst for carrying outsaid chemical reaction; recovering, reacted solution at the top portionof said reactor, filtering said solution by passing it successivelythrough at least one primary filter and at least one secondary filter;recovering filtered reacted solution which has been thus rid of saidcatalyst; sequentially backwashing said at least primary filter with apulsed flow taken from at least one of said filtered reacted solutionand the supply solution to be reacted; wherein said backwashing returnsthe catalyst retained on said at least one primary filter into saidreactor; and recirculating a fraction of unfiltered reacted solutionlocated towards a bottom portion of said reactor, at a first flow ofunfiltered reacted solution which is greater than a second flow thereofwhich supplies said reactor with solution to be reacted; saidrecirculation being provided by stirring inside said reactor and by saidfraction of unfiltered reacted solution being redirected, for saidrecirculation, at said at least one primary filter and thus providingfor tangentially skiming over said at least one primary filter. 2.Method according to claim 1, wherein said first flow for recirculatingunfiltered reacted solution is 3 to 10 times greater than said secondflow for supplying said reactor with solution to be reacted.
 3. Methodaccording to claim 1, wherein said stirring, inside said reactor isoperable for: dispersing the gas and the catalyst within said solutionto be reacted; a pumping effect; an internal recirculation of saidsolution to be reacted.
 4. Method according claim 1, wherein said gas ishydrogen (H₂).
 5. Method according to claim 1 operable for the carryingout hydrogenation of at least one derivative of anthraquinone in thepreparation of hydrogen peroxide (H₂O₂).
 6. A device which is suitablefor carrying out the method according to claim 1, comprising: a reactorhaving a top portion and a bottom portion and an outlet a solutionsupply, a gas supply and a device for delivering reacted solution whichhas been reacted in the reactor; said solution supply and said gassupply are arranged at H said bottom portion of said reactor, whereassaid device for delivering reacted solution is arranged said topportion; a filtering assembly mounted at the outlet of said reactor, andon said device for delivering reacted solution said filtering assemblyincluding at least one primary filter and at least one secondary filterthat follows said primary filter; a backwash device for carrying outsequential backwashing of said at least one primary filter with a pulsedflow, wherein said backwash device is operable to take its supply ofliquid for backwash from at least one of downstream of said filteringassembly from filtered, reacted solution and upstream of said reactorfrom solution to be reacted; a recirculating device for recirculating aflow of reacted solution, with tangential skimming over said at leastone primary filter, towards said bottom of said reactor and said stirrerof said reactor provide, for said recirculation.
 7. Device according toclaim 6, wherein said stirrer comprises a multi-stage stirrer.
 8. Deviceaccording to claim 7, wherein said stirrer comprises: a dispersiondevice, which operates in said bottom portion of said reactor; a middlestage operable to provide for internal recirculation inside said reactorand to help to pump reacted solution towards said at least one primaryfilter; and an upper stage, which pumps said reacted solution towardssaid at least one primary filter.
 9. Device according to claim 8,wherein said middle stage and said upper stage include blades which areinclined downwards and upwards, respectively.
 10. Device according claim6, arranged in a hydrogen peroxide production loop (H₂O₂).